[stars] Red Supergiants

How big can these stars get, Also can a star be born at a Supergiant level?

I'm having a hard time understanding a particular object. The Helix Nebula is said to be the result of a supernova explosion, a closeup reveals tiny knots of gas, now these knots are said to be at least the size of our solar system, which means that the star dying to cause the Helix Nebula must've been huge, but how big was this star?

Based on current knowledge [which is not precise or complete] red giants evolve from HR main sequence stars. The core temperature necessary to inflate the photosphere to red giant proportions cannot be achieved by hydrogen fusion. Perhaps a star formed predominantly from helium could theoretically be born a red giant, but I doubt it could ever get hot enough to initiate helium fusion without blowing itself apart.